1. Field of the Invention
The present invention relates to a method and apparatus for producing an integral skin polyurethane foam having a non-porous, solid layer on its surface. The method and apparatus are suitable for the production of polyurethane foams by so-called reaction injection molding (RIM hereinafter). Examples of the polyurethane foams include automotive parts such as steering wheels, steering wheel pads, instrument panels, console box lids, glove box lids, headrests, armrests, and air spoilers.
2. Description of the Prior Art
Among polyurethane foams is an integral skin semi-rigid polyurethane foam. It is produced in the following manner from a long-chain polyol having a molecular weight of the order of thousands, a low-molecular polyol having a molecular weight of the order of tens to hundreds, a catalyst, a pigment, a blowing agent (trichlorofluoromethane or freon-11 or simply freon hereinafter), and an isocyanate. All the components except isocyanate are preliminarily mixed to give a polyol mixture. The polyol mixture is mixed with the isocyanate by a mixing head in a prescribed ratio to give a reacting mixture. Finally, the reacting mixture is injected into a mold cavity made of a highly heat-conductive material, so that it expands in the mold cavity.
Upon injection into the mold cavity, the polyol mixture and isocyanate undergo the urethane-forming reaction, which is exothermic enough to vaporize the freon in the reacting mixture into a myriad of bubbles. Thus the reacting mixture slowly expands and flows in the mold cavity, expelling the air from the vent hole of the mold cavity when it has reached the vent hole.
At the center of the reacting mixture, the urethane-forming reaction and the vaporization of freon take place simultaneously, sustaining a myriad of freon bubbles. The consequence of the reaction is the formation of a highly expanded core at the center of the mold cavity. On the other hand, the reacting mixture close to the walls of the mold cavity undergoes the urethane-forming reaction more slowly than that at the center because the reaction heat dissipates through the walls of the mold cavity. The slower the reaction, the less the reaction heat. This suppresses the vaporization of freon and hence generates a less number of freon bubbles. Freon bubbles are compressed or collapsed by the internal foaming pressure. The result is the formation of a non-porous, solid integral skin on the surface of the polyurethane foam. Overpacking of the reacting mixture is a common practice to increase the foaming pressure, thereby facilitating the formation of the integral skin, and to promote the mold filling and air vent.
The above-mentioned conventional method for producing integral skin polyurethane foams by the aid of freon poses the following five problems.
(1) Freon as the blowing agent, once released in the atmosphere, destroys the ozono-sphere to aggravate the healthy living environment. For this reason, its use is limited or banned. This situation necessitates the development of a new method for producing polyurethane foams without freon.
(2) Forming the skin by collapsing freon bubbles in the surface section by the internal foaming pressure permits minute freon bubbles to remain inevitably in the skin. They impair the appearance.
(3) Over-packing to increase the internal foaming pressure causes a large amount of the reacting mixture to be discharged from the vent. This leads to a great loss of materials.
(4) An effective way of reducing the cycle time and increasing the production efficiency is to increase the reaction rate of the reacting mixture, thereby reducing the cure time. This object is achieved by increasing the amount of the catalyst or raising the temperature of the reacting mixture at the time of injection. The latter is preferable to the former, which leads to an increase in production cost. The latter method, however, is impracticable because the raised temperature causes freon to vaporize before injection. Moreover, the foaming by freon involves a disadvantage that the heat of reaction of the reacting mixture is used as the heat of vaporization of freon. This retards the temperature rise of the reacting mixture and unduly prolongs the cure time.
(5) The conventional foaming by freon suffers from another disadvantage in the case where a core is placed in the mold cavity. That is, the core brings about the turbulence of the reacting mixture, which leads to defects such as pinholes, voids, and short shot due to entrapped air. This trouble is conspicuous in the production of steering wheels, in which case a long ring core is placed in the mold cavity. Thus it is necessary to select an adequate place for gating to minimize turbulence and to form several vents to release entrapped air.
In the meantime, methods of producing polyurethane foams by using reduced pressure are disclosed in the following Japanese Patents. However, they do not necessarily disclose or suggest the object, constitution, and effect of the present invention.
(1) Japanese Patent Laid-open Nos. 63237/1980 and 63238/1980 disclose a method for causing the reacting mixture to uniformly expand to fill the mold cavity completely, by evacuating the mold cavity through thin grooves formed in the walls of the mold cavity. The disclosed invention is intended solely to produce a uniform rigid polyurethane foam free of defects such as voids and blisters. Therefore, it mentions nothing about the blowing agent.
(2) Japanese Patent Laid-open No. 111648/1981 discloses a method of foaming the reacting mixture in an atmosphere of reduced pressure. It claims that this method produces an effect of reducing the amount of the blowing agent (such as freon) required. However, this method employs freon as with the conventional one, and hence it also poses the problem associated with the destruction of the ozono-sphere. Therefore, it is basically different from the present invention which does not resort to freon at all.
(3) Japanese Patent Laid-open No. 164709/1987 discloses a method for producing a low-density polyurethane foam by expanding a reacting mixture (containing water as a blowing agent) in an atmosphere of reduced pressure. Unlike the present invention, this method is not intended to produce an integral skin polyurethane foam. (The formation of a skin is not desirable.) It follows from the fact that water is used as a blowing agent that this method does not teach or suggest the present invention in which no blowing agent is used.
(4) Japanese Patent Laid-open No. 268624/1988 discloses a method of producing a polyurethane foam by causing a reacting mixture to capture nitrogen gas as much as 2-30 vol % in the form of fine bubbles dispersed therein, and expanding the reacting mixture in an atmosphere under reduced pressure. This method is not designed to produce the integral skin polyurethane foam as in the present invention. It follows from the fact that dispersed fine bubbles of nitrogen gas are used as a blowing agent that this method does not teach or suggest the present invention in which no blowing agent is used.
(5) Japanese Patent Publication No. 5528/1989 discloses a method for producing a rigid polyurethane foam by injecting a reacting mixture into a mold cavity evacuated to about 50-500 mmHg at a packing ratio of about 150-450%, and subsequently expanding the reacting mixture while keeping the mold temperature at about 10.degree.-45.degree. C. It follows from the fact that the reacting mixture contains a blowing agent (such as freon) that this method does not teach or suggest the present invention in which no blowing agent is used.